Right all of you who are stuck doing the car sell glitch, let me help you.
Go into an invite only session, make sure all PV's are in your garage.
Get in any car you like, then ring Simeon (can ring Ron or any other character that makes a pop-up screen appear)
Drive to the garage doors at Weazel Plaza and push against them with the car, MAKE SURE YOU LEAVE A TINY GAP SO THE GARAGE DOOR CAN OPEN A TINY BIT BUT NOT FULLY, get out the car then hang up the phone.
Drive out the car you want to sell, then when you spawn outside, wait for a little yellow circle to spin in the bottom right (if it dont appear, try again)
Once thats happened, crash on the way to cause some damage to the car, drive to the LSC doors till they open and put the 2 front wheels in, with the back of the car out of LSC, call the mechanic and drive in to LSC, order ANY car (you dont need to buy a rebel or anything), then press right on the dpad, click sell, then click the value, then wait till the little orange circle appears again, once it's gone, sell the car.
Go outside and get in the PV you ordered, call MMI and get the original car back, then drive your PV into your garage, keeping your original car and keeping the car you ordered. Then quit your session and rejoin an invite only again, and repeat.
This method works 100% with me at the moment. (on PS3)
You do not need to purchase any vehicle online whilst doing this, and you still get the full price for selling it.
This is working 100% for me on the xbox at Eclipse towers and it's not finnicky at all with a few notes/modifications.
1) Before you do anything, leave online and start a new game. Then go to an invite only session when you can. This is what fixed my car not getting destroyed.
2) Instead of stealing any car, order the Stretch from Pegasus. It spawns down the street from Eclipse and won't despawn or fail to destroy your vehicle like a random car can. This makes calling Simeon or anyone else unnecessary as well.
3) Leave no gap when jamming the stretch against the doors. Just do it head on instead of parking it sideways like some videos show.
4) You can call Mors Mutual to make sure you can claim insurance on the vehicle once you exit your garage, but don't claim it yet. I don't press the gas at all, I don't know if that matters (probably not).
5) Quitting and rejoining a session is unnecessary.
6) Keep in mind that the vehicle you call for before entering LSC will no longer have insurance.
7) I did not buy the Rebel, and I got the 48 minute timer. As I'm doing this right now, I won't be able to see if that stops the timer or what is doing that for at least an hour. Update: I thought buying a rebel fixed this but sometimes I get the timer and sometimes I don't.